`generate.go branch` does not fork explicitly referenced images

[howardjohn]

Some jobs have per-job image overrides. When we branch, these are ignored.

We should either automatically update these, or maybe at least warn about them so they can be addressed.

cc @istio/release-managers-1-9

[ericvn]

cc @istio/release-managers-1-10 This was one of the noted items during the branching.

[Monkeyanator]

cc @brian-avery who I followed up with on the specifics of the issues we had with the generate step